Trailers used in the heavy duty trucking market are frequently fitted with interior cargo lights to provide drivers with enough light to safely move around the inside of the trailer when the vehicle is parked. The interior cargo lights are typically powered from the vehicle's alternator via a junction box on the front of the trailer. The typical output of a tractor trailer alternator is around 14 VDC, but the voltage available at the junction box is typically no higher than around 12.5 VDC. There exists a problem in today's market to get as much of that voltage to the interior cargo lights as possible. When the vehicle is in motion, however, the power supplied by the junction box blue wire is mandated as the power supply to the trailer's antilock braking system (ABS) control module.
In order to supply power to the interior cargo lights only when the vehicle is not in motion, prior systems insert a pressure controlled switch into the trailer's emergency air brake line. When there is no pressure in the emergency air brake line, the parking brakes are engaged, and the pressure controlled switch allows power from the junction box to be supplied to the interior cargo lights. The pressure controlled switches used in prior systems, however, are high current devices that cause a drop in the voltage available from the junction box, translating to less voltage that is available to supply the interior cargo lights. It would be advantageous, therefore, to provide a means for supplying power from the junction box to the interior cargo lights that eliminates the high current switch used in prior systems.